Contemporary signal processing systems often use filters to remove undesirable information from signals to improve signal fidelity. In digital signal processing schemes it is often desirable to apply a median filter to process signals. Median filters are non-linear filters that can be used to achieve dramatic results in diverse subtractive synthesis applications such as in video, speech, general acoustics, radar, and other uses of signal processing.
In particular a median filter may be applied to significantly improve signal-to-noise ratio in internal combustion engine performance monitoring and control applications. One such application is misfire determination. In this application an engine's combustion performance is monitored by observing, for example, acceleration of the engine's crankshaft via an accelerometer. In an ideal situation, if a cylinder of the engine under observation misfires (all of the air-fuel mixture charged into the cylinder doesn't completely combust) the engine's crankshaft will tend to decelerate rather than accelerate as expected. If this deceleration is observed by a misfire detection system, then a misfire can be indicated.
In a typical application however there are many sources of noise that contaminate the observation process. For one, since the engine is mechanically coupled to a transmission, which in turn is coupled to a drive shaft and wheel assembly, as a vehicle powered by the engine traverses, a bump in the road surface can cause a decelerating disturbance to the wheel assembly. Through the above-described coupling mechanism the disturbance can be reversed coupled back to the engine's crankshaft--thus the accelerometer. Some form of signal fidelity improvement is necessary to eliminate this source of noise to prevent a false misfire indication. Since this source of noise causes a signal to be generated at a characteristically very low frequency the signal processing mechanism could use a traditional digital high-pass filter to filter out these disturbances. Practically this would require a very high order digital filter because the signal spectra generated by misfiring behavior by the accelerometer is closely proximate the signal spectra associated with the road surface induced signal.
A median filter fits well into this type of application for eliminating the road surface induced signal from the misfire related spectra generated by the accelerometer. The median filter achieves a similar result to a very high order digital high-pass filter with a relatively low order structure.
Digital or sampled data type median filters function by operating on a fixed-length list of data samples. The median filter determines which sample present on the fixed-length list represents a value arithmetically median within the list. Some prior art median filters used a storage memory intensive scheme for resolving the median value within a list of data samples. For instance, once a list is filled with data it is sorted into an arithmetically ordered list and the data sample positioned in the center of the list becomes the median result. By subtracting the median result from the time ordered data sample positioned at the center of the time ordered list a median filtered data sample can be synthesized. This can be shown more clearly with an example.